We had an RP that actually survived the guildfall, but it died some months afterwards with two core players being mostly unavailable and certain blatant design flaws rearing their heads. We can't give good answers to exactly what'll keep an RP alive, but we can give some ideas to what can kill them.

The RP we had was Star Wars-themed, with captive Jedi during the Cold War between Vitiate's Sith Empire and the Galactic republic following the Sacking of Coruscant. The one design flaw that most certainly killed that RP, was that the Jedi had no real hope. Since the RP was based around players being on both sides of the conflict, that lack of hope truncated the options the Jedi faction had for doing anything. The prison ship was designed with a few too many of the Evil Overlord list points taken into account, so they had almost no chance to get to the ship's hangar, let alone get away from it. (when not in hyperspace, the ship was escorted by two Terminus-class destroyers. as well as being heavily armored and armed on its own). As such, even outside hyperspace, the ship was essentially unassailable. 

On the ship, the prisoners essentially had the options of falling to the Dark Side or dying. They were often tortured, but you can get tired of writing imaginative torture after a while. Especially if the RP restricts you from using certain forms of torture.

You can probably see what we're hinting at here... If you're designing an RP with two playable factions opposing each other, you'd better make sure that both the factions have a full range of options. Don't let your personal preference for one faction dominate the RP. It only comes back to bite you later.

We've long had a plan to bring that RP back to life, so we've made some plans we think others could use too in order to make an RP work better in the long run... Not sure how good they are, but you never know. One plan we have for the second iteration of said RP, is to have multiple GMs, with different fields of "duty". One GM for each faction, one GM for the RP mechanics. We also plan to have fewer restrictions on what we permit the players to do in the RP. PCs should not be immortal.

However, another trait we've found useful for RPs, especially the complex ones like the one we've described here, is that we're strict on the Character Sheets. As it was in an existing universe, we have lore to stick to. Forcing people to follow certain rules when they create characters helps keep the chaff away from the wheat. Star Wars, for example, is a massive universe. You can't fairly cover more than a fraction of it in one RP. Thus players need to have restrictions when they create their characters. Our chosen prime restriction in the above RP was that players can only play Sith/Jedi. No regular soldiers, no bounty hunters, no Mandalorians and so forth. Sure, this removes some potential players, but it makes the whole manageable. When such roles are needed for whatever, we have our players use NPCs. Like the player characters, these are mortal and can die. Unlike the player characters, not quite so much is needed to kill an NPC. 

Another important element that is easy to forget is the classic rule of "never split the party". If the PCs are too far apart, you can easily end up with the RP stagnating as various players are waiting for specific other players. The above RP had that flaw in having several cell blocks for the prison complex. While that makes sense containment-wise, it restricts interaction between the characters, which is a bad thing.

The final thing we think can help an RP survive long is permitting players to introduce their own plot lines. The GM(s) shouldn't have exclusive right to making the plot. Often players will have excellent ideas that improve the RP in the long run. Granted, the GM(s) should always have a certain control in place so that the RP isn't completely taken off the network of tracks, but He/she/they should not try to keep the RP on a single track. That would be railroading, and that is not a fun thing to go through.